BOATS

Although boats float, their weight makes them settle a
little way into the water. This means they create some resistance or drag when
they move through the water. The bow (front) of a boat is V-shaped and curved.
This raises the boat up as it goes faster, and helps reduce drag. Boats are
powered by sails, oars, or engines that turn propellers at the rear. The
propellers push water backwards, and this backward thrust moves the boat
forwards.

HYDROFOIL

A hydrofoil is a high-speed boat that seems to fly along almost out
of the water. The boat has small underwater wings called foils. These work like
the aerofoil wings of an aeroplane. As the boat speeds along, the foils
generate a force upwards that lifts its hull (body) clear of the water. This
reduces drag and helps the hydrofoil go faster.

SUBMARINES

Submarines can float on the sea, sink just beneath the
surface, or dive to the seabed. They dive or surface using tanks that work like
a fish’s swim bladder. When the tanks are filled with water, the
submarine dives. When they are filled with air, it surfaces. A submarine can
select its level in the sea by changing the mixture of air and water in its
tanks.